Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such networks, which are usually multiple access networks, support communications for multiple users by sharing the available network resources. One example of such a network is the UMTS Terrestrial Radio Access Network (UTRAN). The UTRAN is the radio access network (RAN) defined as a part of the Universal Mobile Telecommunications System (UMTS), a third generation (3G) mobile phone technology supported by the 3rd Generation Partnership Project (3GPP).
One of the more recent enhancements made available in a UTRAN relates to multi-antenna technology, where multiple transmit antennas and/or multiple receive antennas can be used in a wireless link to enhance data transmission performance, enabling diversity gains to reduce multipath fading and increase transmission quality, as well as spatial multiplexing gains to increase data throughput.
Spatial multiplexing may be used to transmit different streams of data simultaneously on the same frequency. The data steams may be transmitted to a single receiving device to increase the data rate, or to multiple receiving devices to increase the overall system capacity. This is achieved by spatially precoding each data stream, and then transmitting each spatially precoded stream through a different transmit antenna. The spatially precoded data streams arrive at the receiving devices with different spatial signatures, which enables each of the receiving devices to recover the one or more the data streams destined for that device.
Spatial multiplexing may be used when channel conditions are good. When channel conditions are less favorable, beamforming may be used to focus the transmission energy in one or more directions, or to improve transmission based on characteristics of the channel. This may be achieved by spatially precoding a data stream for transmission through multiple antennas. For example, for downlink transmissions, to achieve good coverage at the edges of the cell, a single stream beamforming transmission may be used in combination with transmit diversity.
Within the field, the term MIMO (multi-input, multi-output) is frequently used to refer to spatial multiplexing, while beamforming, which is another multiple antenna technology, might be excluded from the conventional definition of MIMO. However, within the present document, the term MIMO is used broadly to refer to all such multi-antenna technologies, not only including spatial multiplexing, but also including beamforming.